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|duration = 12 seconds 〔 |magnitude = 6.5–6.7 Mw 〔〔 |depth = 〔 |location = |type = Oblique-slip |countries affected = Greater Los Angeles Area Southern California United States |damage = $553 million 〔 |intensity = XI (''Extreme'') 〔 |PGA = 1.25''g'' at Pacoima Dam |landslide = Yes |casualties = 64 killed 〔 }} The 1971 San Fernando earthquake (also known as the Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unanticipated thrust earthquake had a moment magnitude of 6.5 or 6.7 (as determined by several independent institutions) and had a maximum Mercalli intensity of XI (''Extreme''). The event was one in a series that affected the Los Angeles area in the late 20th century, and a study of the Sierra Madre Fault during that time indicated that more substantial thrust earthquakes had occurred near the Transverse Ranges in the past. Damage was locally severe in the northern San Fernando Valley, and surface faulting was extensive to the south of the epicenter in the mountains, as well as urban settings along city streets and neighborhoods. Uplift and other effects affected private homes and businesses. The event impacted a number of health care facilities in Sylmar, San Fernando, and other densely populated areas north of central Los Angeles. The Olive View Medical Center and Veterans Hospital both experienced very heavy damage, and buildings collapsed at both sites, causing the majority of deaths that occurred. The buildings at both facilities were constructed with mixed styles, but engineers were unable to thoroughly study the buildings' responses because they were not outfitted with instruments for recording strong ground motion, and this prompted the Veterans Administration to install seismometer Transportation around the Los Angeles area was severely afflicted with roadway failures and the partial collapse of several major freeway interchanges. The near total failure of the Lower Van Norman Dam resulted in the evacuation of tens of thousands of downstream residents, though an earlier decision to maintain the water at a lower level may have contributed to saving the dam from being overtopped. Schools were affected, as they had been during a previous earthquake in Long Beach, but this time amended construction styles improved the outcome for the thousands of school buildings in the Los Angeles area. Other aspects of the event included a methane seep that emanated from the floor of the Pacific Ocean near Malibu for several days, and hundreds of various types of landslides that were documented in the San Gabriel mountains. As had happened following other earthquakes in California, legislation related to building codes was once again revised, with laws that specifically addressed the construction of homes or businesses near known active fault zones. == Tectonic setting == The San Gabriel Mountains are a long portion of the Transverse Ranges and are bordered on the north by the San Andreas Fault, on the south by the Cucamonga Fault, and on the southwest side by the Serra Madre Fault. The San Bernardino, Santa Ynez, and Santa Monica Mountains are also part of the anomalous east–west trending Transverse Ranges. The domain of the ranges stretches from the Channel Islands offshore, to the Little San Bernardino Mountains, to the east. The frontal fault system at the base of the San Gabriel Mountains extends from the San Jacinto Fault Zone in the east to offshore Malibu in the west, and is defined primarily by moderate to shallow north-dipping faults, with a conservative vertical displacement estimated at . Paleomagnetic evidence has shown that the western Transverse Ranges were formed as the Pacific Plate moved northward relative to the North American Plate. As the plate shifted to the north, a portion of the terrane that was once parallel with the coast was rotated in a clockwise manner, which left it positioned in its east–west orientation. The Transverse Ranges form the perimeter of a series of basins that begins with the Santa Barbara Channel on the west end. Moving eastward, there is the Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, with active reverse faults (San Cayetano, Red Mountain, Santa Susana, and Sierra Madre) all lining the north boundary. A small number of damaging events have occurred, with three in Santa Barbara (1812, 1925, and 1978) and two in the San Fernando Valley (1971 and 1994), though other faults in the basin that have high Quaternary slip rates have not produced any large earthquakes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1971 San Fernando earthquake」の詳細全文を読む スポンサード リンク
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